This invention relates to pH electrode glass compositions. More particularly, this invention relates to pH electrode glass compositions having very low bulk resistivities and very low surface resistivities after aging in aqueous solutions.
Glass electrodes sensitive to the hydrogen ion activity, or pH, of a solution have been known for some time. Since their discovery in 1906 and the various modifications and improvements in glass compositions which followed, such electrodes have come to play an important role in both research and industry.
Desirable properties in a pH glass electrode, which largely are functions of the glass composition and configuration, include the following:
a. Low volume or bulk resistivity. Potentiometric error due to offset current and finite impedance at the input of the electrode potential measuring means, e.g., a pH meter, is directly proportional to the total resistance of the pH electrode. Consequently, low bulk resistivity in a pH glass allows fabrication of smaller or thicker membranes for such purposes as smaller size, increased strength, and attainment of required geometries, while retaining the good performance of the more common larger or thinner membranes.
b. Low surface resistivity. Decreased surface resistance reduces the tendency of pH electrodes to polarize. The term "polarize" is commonly used to describe the long-term disturbance of pH electrode potential often induced by a brief current flow through the sensing glass membrane. Because recovery from such disturbances typically is slow, for a period of time after a polarizing disturbance the pH measurement shows a slowly decaying error. The practical advantages of decreased surface resistance include enhanced speed of response, especially with high capacitance cables, faster decay of transient potentials induced by external electrostatic coupling, and faster stabilization after connection to the pH meter and after changing bathing solutions. These advantages are particularly important at low temperatures and with noncombination electrodes which do not have a low-impedance reference junction in the vicinity of the pH electrode to act as a sink for stray currents.
c. Near-theoretical slope (volts/pH unit). Maintenance of near-theoretical slope over the entire pH range of 0-14 simplifies calibration and allows pH measurements over the maximum range usually encountered.
d. Low sodium error. Low sodium error improves the accuracy of pH measurements in strongly alkaline solutions containing sodium ions.
e. Low asymmetry potential. Asymmetry refers to the potential difference across a membrane when the inner and outer surfaces are bathed with identical solutions. A low asymmetry potential generally is associated with enhanced stability and uniformly in the potentiometric characteristics of manufactured electrodes.
f. High chemical durability. The pH glass should have sufficient chemical durability to allow a long life in strongly acidic or alkaline solutions.
In general, it is not possible to obtain a single pH glass in which all of the desirable properties have been optimized. There usually is a degree of compromise associated with any pH glass electrode, with the end use or uses dictating which properties are of the greatest importance. For example, the more recent pH glasses emphasize such properties as low sodium error, low bulk resistivity, improved workability, enhanced durability, and the absence of devitrification or phase separation. The most significant of these more recent pH glasses are described below.
U.S. Pat. No. 3,372,104 broadly discloses pH electrode glass compositions which are lithia-silicate glasses provided from a pre-melt composition or mixture according to the following formula, expressed in ranges of mole percentages on the oxide basis: (1) about 27-29 mole percent Li.sub.2 O, (2) about 2-4 mole percent of at least one material selected from the group consisting of Cs.sub.2 O and Rb.sub.2 O, (3) about 4-7 mole percent of at least one rare earth metal oxide, (4) about 1-3 mole percent UO.sub.2 and/or about 1-3 mole percent Ta.sub.2 O.sub.5, and (5) the balance SiO.sub.2 which typically is about 58-63 mole percent. The only rare earth metal oxide actually used was La.sub.2 O.sub.3.
Glasses similar to the above are claimed in U.S. Pat. No. 3,410,777 and have a composition consisting essentially of about 27-29 mole percent Li.sub.2 O, about 2-4 mole percent of at least one material selected from the group consisting of Cs.sub.2 O and Rb.sub.2 O, about 4-7 mole percent of at least one rare earth metal oxide, about 1-3 mole percent UO.sub.2, and the balance SiO.sub.2. Although the only rare earth metal oxide actually used was La.sub.2 O.sub.3, the rare earth metal oxide can be selected from the group consisting of La.sub.2 O.sub.3 and Pr.sub.2 O.sub.3.
Finally, U.S. Pat. No. 4,028,196 discloses pH electrode glass compositions consisting essentially of 30-40 mole percent Li.sub.2 O, 50-60 mole percent SiO.sub.2, 2-8 mole percent La.sub.2 O.sub.3, 2-8 mole percent Ta.sub.2 O.sub.5, and 0-3 mole percent Cs.sub.2 O, wherein the sum of the mole percentages of Li.sub.2 O and Ta.sub.2 O.sub.5 is equal to or greater than 34.
Glass compositions such as those described above have proven satisfactory for the construction of pH glass electrodes having a general utility. For miniature, rugged, or flat-membrane pH glass electrodes, however, there still is a need for glass compositions having very low bulk and surface resistivities and, as a consequence, a reduced tendency to polarize.